There is a thread on this, but it's locked. I would also like to change the date on comments, has this fix been included yet? Or can I get my hands dirty in PhpMyAdmin for the moment? I'm migrating a website for a client (from Experssion Engine), they would like to keep the comments but there's no easy migration tool for this, I don't think.
It can be done easily in PhpMyAdmin if users have access to this from cPanel on their websites.